There are many examples of industrial fabrication processes that use a continuous moving sheet of material to improve the efficiency and reduce the cost of the final product. There are what are known as cast films, such as polycarbonate, extruded films, such as polyethylene, paper products, and various composite materials such as foil backed paper. There are also many industrial fabrication processes that use a continuous moving sheet of a substrate upon which one or more coatings are applied. Examples, of coated materials include waxed paper, polyethylene coated cardboard, and polarized films.
In each of the above noted manufacturing situations there are process control values that must be maintained in order to efficiently make the desired final product. An example of such a process control is maintaining a specified range of thickness for a plastic coating on a fibrous substrate for waterproofing. The plastic film, for example polyethylene, must be of sufficient thickness to have high integrity, but must be thin enough to be cost and weight effective. Therefore, it is known to use infrared (i.e., IR) spectrometry to measure the thickness of the plastic layer by means of measuring either the transmitted or the reflected intensity and computing a measure of relative intensity such as transmission, reflection or absorption of known IR peaks. The calibration constants developed from using these known IR peak intensities on samples of the same material composition of known thickness may then be used to make quantitative measurements on a material as the material is being manufactured.
A problem with this method is that the intensity value of the peak depends in art upon the composition of the substrate material, and so the calibration standards and constants are based upon a specific substrate composition. The substrate material may be a liquid base material for cast films, a sheet material for coating operations, or a solid material to which a material property transformation will take place during the manufacturing process. If the substrate composition changes, either because of a lack of process control at the substrate manufacturer, or because of a change in substrate source during a substrate roll splice, or even a change in humidity under which the substrate was stored, then the apparent thickness of the coating material may change because the calibrated intensity peak may now be different. It may also occur that even the same composition substrate material may have different IR intensity values when manufactured in different plants and under differing conditions. Thus it would be ideal to require that the rolls of the substrate composition that are spliced together during the coating process to provide the substantially continuous moving sheet of substrate material being coated, all be selected from a single manufacturer in order to minimize the calibration problem. However, this practice may present a warehousing and inventory supply problem, and may thus result in reduced efficiency manufacturing.
A similar problem exists in cast film manufacturing where a change in, for example, the molecular weight of a feed stock material may cause the calibrated value for an IR spectrometer reading of the continuous output sheet of dry film to drift, resulting in an incorrect reading, a consequent on-line change in some processing parameter such as casting speed in an attempt to fix the incorrect thickness, and a resulting incorrectly manufactured actual film thickness. Such problems in both coating and cast film manufacturing processes may result in factories that produce defective products for long periods of time, i.e., an entire day, resulting in financial loss.
As noted above, the calibration standard and constants used must be selected based upon the specific base or substrate material, and upon the coating material. Thus a processing line that manufactures plastic coating on paper products may require new calibration constants when the product being made changes, with consequent opportunity for lost time, increased work load and error potential. For example, in manufacturing plastic coatings on a matte finish paper, a specific set of calibration constants will have been determined and used. When manufacturing an otherwise identical product that coats the same plastic thickness on a shiny slick (as opposed to matte finish) paper substrate, the correct calibration may be different. The calibration constants may even have to change while using a single type of paper finish if the brightness of the printed color on the paper changes, such as may occur when the printing press reservoirs run low. It would improve manufacturing efficiency and reduce potential defective product to have an automatic method of adjusting calibration.
In many continuous moving sheet types of manufacturing processes, it is not sufficient to only measure the property of interest at a single point on the material. There is a benefit in making periodic measurements during manufacturing since the process parameters may change with time. There may also be a need to measure the property of interest at various points across the sheet width to account for spatial variation. In each one of these situations it is possible that the calibration values may have to change as the substrate composition changes in order to obtain accurate physical property values, i.e., for example thickness from the spectrometer readings. This is not possible using the present method of basing the measurements on a single set of calibration constants, or at best a limited number of sets, taken on a piece of the substrate material in a laboratory and applied thereafter for the entire manufacturing run of the particular material/substrate combination.
Another factor in the calibration problem is that the intensity at the wavelength peaks used to make the on-line quantitative measurements may be sensitive to slight variations in the substrate, such as differences in printed colors as previously noted, or differences in the amount of clay like material added to paper to increase the surface gloss and improve print quality. Substrate sensitivity also exists in the two wavelength ratio method. What is needed in the art is a substrate independent method of automatically selecting the correct set of calibration constants in order to accurately measure the property or characteristic of interest on a material.
A substrate independent method and computerized apparatus for on-line analysis of a continuous sheet of material is presented, using illumination of a material with a portion of the radiation spectrum. The method of measuring or evaluating a physical, chemical or atomic property, or characteristic, comprises the steps of using a qualitative analysis to determine the base material type, or select a set of calibration values, then using a quantitative analysis with the selected calibration to determine the value of the property or characteristic. The arrangement uses the entire spectrum portion in the measurement and not a few known absorption peaks. In a preferred embodiment of the invention, the spectrum used is in the infrared spectrum, but other radiation spectrums may also be beneficially used, such as x-ray, ultraviolet, Raman scattering radiation, or nuclear magnetic spin resonance radiation. A preferred detector is an IR spectrometer. First, a calibration is selected by using a radiation detector to measure the radiation intensity, absorbance or reflectivity, at each of a series of wavelengths scattered throughout the spectrum range. The measured intensity is used to calculate a spectral shape or pattern over the spectrum region. This measurement may be made either on a bare substrate, on a coated substrate or at both locations. The measured pattern is matched against stored spectral patterns of possible substrates in a spectral shape library, and the most closely matching spectral pattern is chosen. This qualitative spectrum identification of the substrate type selects the correct set of calibration standard values. Using the calibration values or constants, the same IR system and method may be used to make a quantitative measurement of a property, such as cure percentage, or film thickness, of the material at a variety of locations and times, depending on desired accuracy requirements. In general, the measured physical, chemical or atomic property or characteristic values will be stored in a memory and statistical calculations done. An alarm will be sent if the measured value exceeds a predetermined range, or if the qualitative match is not within a predetermined range.
In another embodiment of the invention, the measurement wavelengths will be evenly spaced to cover a preferred range of 1.3 to 3.4 microns, and the number of wavelengths used will be greater than 10 depending upon the accuracy desired for the spectrum pattern matching.
In still another embodiment of the invention, the pattern matching step is performed by a point by point comparison of the measured intensity at each wavelength versus the intensity of each of the stored spectral patterns at the respective wavelengths, calculating a difference value at each wavelength, and calculating the standard deviation for the difference values for each one of the library shapes. The best pattern match is the one with the lowest calculated standard deviation. Other methods of pattern or shape matching may be used including least squares comparisons, root mean square, correlation, etc.
In another embodiment of the invention, the measured physical property is the thickness of a coating on at least one side of the substrate material, and in another the substrate material is a composite material having multiple bonded layers, for example bonded fibrous material such as paper, or fibrous material having an inorganic or a metallic material coating.
In yet another embodiment of the invention, the selection of calibration value step is either performed after a coating has been applied to the substrate, or the calibration selection step is performed both prior to and after the coating. In another embodiment the calibration selection step is performed periodically, or at splice locations where rolls of feedstock are connected to form a continuous layer.
In still another embodiment of the invention, the quantitative physical property measurements are taken essentially continually over substantially the entire width of the continuous sheet of material.
In general, the arrangement comprises an automatic qualitative evaluation of which of a group of possible substrate spectral shapes is most similar to the measured spectral profile, and then using the selected calibration constants to make quantitative measurements of one or more material properties, such as thickness. With such an arrangement the periodic re-calibration of a process control measurement may avoid the errors caused by unannounced changes in the substrate, and process drift and out of specification waste material may be reduced.